A considerable problem in the area of wind turbines is that of damages caused to wind turbine structures during periods of high loading due to extreme wind conditions. As wind turbine blades are normally prevented from rotating in such conditions, the peak lift forces or peak drag forces which are experienced by wind turbine blades during high wind loads are transferred to the wind turbine tower structure, resulting in considerable stresses and strains experienced by the structure itself.
The lift force L produced by an airfoil in a specific airflow condition is related to the area of the airfoil and to the angle of attack of the oncoming airflow (i.e. the angle between the chord line of an airfoil and the vector line of the oncoming airflow) by the following formula:L=½ρν2ACL where ρ is the air density, ν is the true airspeed, A is the planform area, and CL is the lift coefficient for the angle of attack of the incoming air. The drag force for an airfoil is produced by a similar formula, where CL is replaced by Cd, the drag coefficient.
With reference to FIG. 1, the lift coefficient (indicated by the dashed line) and the drag coefficient (solid line) are shown for a standard airfoil, plotted against different angles of attack. As can be seen from the graph, for a standard airfoil shape the maximum lift force generally occurs at an angle of attack of the oncoming wind of approximately 15°-20°. The maximum drag force occurs at an angle of attack of approximately 90°.
With reference to FIG. 2, a pair of illustrative drawings of a wind turbine blade 10, tower 12, and nacelle 14 are shown, demonstrating the high loads which are experienced during extreme wind conditions (for ease of reference, the blade 10 is shown as being in line with the tower structure 12). In FIG. 2(a), the angle of attack (AoA) of the oncoming airflow 16 is approximately 15°-20°. Accordingly, the blade 10 experiences a maximum lift force in the direction indicated by arrow 18. Such a lift force 18 on the blade 10 transfers into a thrust loading on the wind turbine nacelle 14, and subsequently on the tower 12, indicated by arrow 20.
Similarly, in FIG. 2(b), when the oncoming wind (indicated by arrow 22) has an angle of attack (AoA) of approximately 90°, the blade 10 experiences a maximum drag force in the direction indicated by arrow 24. This drag force 22 then results in a corresponding thrust loading on the tower 12 as indicated by arrow 26. Such thrust loading forces 20, 26 contribute greatly to the stresses and strains experienced by a wind turbine structure, and may result in damage or structural failure of the wind turbine when in extremely hazardous loading conditions.
A top plan view of a standard two-bladed wind turbine is indicated in FIG. 3 at 28. The wind turbine 28 comprises a tower 30, turbine nacelle 32 positioned at the top of the tower 30, and a pair of wind turbine blades 34 extending in opposite directions from a hub 33 provided at said nacelle 32. When a two-bladed turbine 28 is stopped rotating (e.g. during an extreme wind condition), the two blades 34 will generally settle into a steady-state horizontal alignment, as indicated in FIG. 3. As described relating to the diagrams in FIG. 2, any strong wind loads (indicated by arrow 36) will act on the entire span of the blades 34, resulting in a corresponding lift or drag force 38 on the blade 34. Such forces 38 acting along the length of the blade 34 result in a cumulative thrust force 40 acting on the turbine nacelle 32 and tower 30, which determines the design load which is set for such a turbine to ensure adequate operation. Any winds in excess of such speeds may result in damage to the turbine structure 28. A perspective view of the forces acting on one of the wind turbine blades 34 is illustrated in FIG. 4. An example of a known three-bladed partial pitch turbine can be seen in US Patent Application Publication No. 2009/0148285 and in 2009/0148291.
US 2009/0148291 disclose a wind turbine having a rotor with three blades. Said rotor with three blades will never be able to find a neutral position in the wind during an extreme wind condition. Therefore it is very common to control the wind turbine in a manner where the outer most part of the blades will be pitched to a feathered position, but as the innermost part of the blades is fixed, this part can not be pitched. In an extreme wind condition the inner blade parts of US 2009/0148291 has the function to guide the wind around the nacelle and in order to do that, the rotor needs to be aligned into the wind. This can only be done by performing a control of the yawing system of the wind turbine, as when the turbine is coupled to the grid and producing power. By operating and controlling the yaw system, the angle of attack is controlled and the wind will be guided around the nacelle. Operating a wind turbine according to US 2009/0148291 will minimize the loads from an extreme wind condition on the outer blades and thus also on the tower and on the foundation, but there will still be some heavy loads from the inner blade parts as they are subject to the full extreme wind load. Further this system will only work as long as it is possible to operate and control the yaw system, which normally can be a problem during extreme wind situations as the wind turbines most often will be decoupled from the grid. If a wind turbine as disclosed in US 2009/0148291 is not controlled and operated into a certain direction in relation the wind direction, very large and unwanted loads will occur. This is actually very crucial as the direction of the wind very often will be changing rapidly and also radically during an extreme wind condition such as a storm or a typhoon. A wind turbine yaw system will not be fast enough to follow such rapid changes in the wind direction and thus very large loads will have to be dealt with.
The International Electrotechnical Commission (IEC), which maintains the standards body for wind turbine design ratings, defines extreme wind conditions as wind shear events, as well as peak wind speeds due to storms and rapid changes in wind speed and direction (www.iec.ch).
If each entire blade 10, 34 of a wind turbine experiences peak lift/drag forces, as such forces are proportional to the square of the wind velocity the subsequent thrust forces experienced by the wind turbine structure 12, 14 or 30, 32 can be significant. For example, IEC Class I turbines must be rated to withstand extreme gusts of 70 meters per second (m/s). It is the constraints of meeting such ratings requirements that force wind turbine manufacturers to design wind turbine structures which are capable of withstanding such loads, e.g. through the use of additional reinforcement materials. Such design features contribute towards the relatively high construction costs of wind turbines.
It is an object of the invention to provide a method of controlling a wind turbine in extreme wind conditions which decreases the effect of loads experienced by the wind turbine structure, and accordingly reduces the costs associated with wind turbine production, as well as the risk of damage to the wind turbine.